how and why warnings fail — e.g., because you want to delete almost everything quickly, except for the things you accidentally delete, #@%@#$

how "undo" is often a better solution — e.g., oh, you can just undo that accidental delete, whew!

This reminded me of a great review of research on the topic of why warnings fail, Boxed risk warnings: research findings, by Ruth Shrensky. This review was done as part of a larger project to revise the book, Writing about medicines for people, by David Sless and Ruth Shrensky, on "definitive guidelines for writing, designing, and testing medicine information for consumers."

(Kind-of makes one wonder what might be possible if medicine came with an "undo button" that allowed one to un-take a pill. . .)

Some people might have heard me refer to an interaction design in-joke from my experience with the surprising glut of so-called usability experts hired in dot com companies—one in particular, who interrupted an intense design problem-solving session with her solution (drawn on a whiteboard) of a "cancel-delete" button / warning / undo interface. (That was, imho, a definite if they have no bread, let them eat cake moment in the history of user centered interaction design.)

I actually can't remember how cancel-delete was even imagined that it might work. But, to be fair, we had a hard design problem involving documents in a workflow that could be canceled (stopped in the workflow) and/or deleted (document erased). I'm thinking we needed something like an undo for "cancel and delete" which would be for a use case like:

the user wants to cancel the workflow and delete the document

undo! the user still wants to cancel the workflow, but un-delete the document

Anyway, because of all of this, I like to tell interaction designers to make sure they include a cancel-delete button in whatever they build! (Sorry IxD folks, I know you're getting tired of hearing that . . .)